The present invention pertains to applications of large battery arrays. Particularly, the invention relates to extended-life battery arrays used in electrically powered vehicles.
In the recent drive to design and produce large electrically powered devices such as electric vehicles for human transportation, a major technical challenge has been the efficient storage of electrical energy. The primary devices for electrical energy storage are primary and secondary electrochemical cells. Past electrochemical cell designs have focused on units having relatively small capacity to be used singly or in small arrays to power devices with relatively small energy requirements. These have been devices using electrical energy for primarily non-motive purposes such as lighting, sound production and signal generation. Until recently, small electrochemical cells have been effective for these purposes. Large motive devices, such as personal automobiles, with large power and energy requirements have introduced significant new design requirements. These include high voltages and drain rates, large total capacity, long life, and high energy density. One consequence of the high drain rates and large size of automotive battery arrays is the generation of large amounts of heat that must be removed from the system. This heat is a result of internal resistive heating in each electrochemical cell. Without means of cooling the cells, the cell temperature can quickly rise to a point that the cell performance is damaged. Depending upon the type of cell, such elevated temperatures may be reached that physical destruction of the cell occurs. Controlling the temperature extremes experienced by cells during operation is necessary to ensure a maximum life is achieved before the cells"" reduced performance requires their replacement. In large arrays of multiple cells it is necessary to control the temperature history of all cells uniformly. Deterioration of a single cell in a cell array, through elevated temperature degradation, can result in significant performance reduction for the entire array. The degrading effect of elevated temperatures on secondary cells is well known in the industry. The temperatures at which degradation becomes significant depends in part on the particular construction of the cell, such as, for example, the material of the separators used between the cell electrodes. For example, in some nickel metal-hydride cells, degradation increases with rising temperature until at about 50 degrees Celsius (xc2x0 C.) the useful life of the cell is significancy reduced. However, in a large array without active cooling, these cells have been found to quickly reach temperatures of 50 to 70xc2x0 C. during operation. In applications such as personal automobiles, the useful lifetime of cell arrays before replacement is desired to be at least 8 to 10 years due to costs and user expectations. This can only be achieved with uniform cooling.
In prior electrical energy storage devices for automotive uses, large combinations, for example a hundred or more, of small secondary cells have been used to provide the necessary power and energy. The selection of small cells is in part to take advantage of existing available cell production to avoid the cost of custom cell fabrication. However, the desire to minimize the impact on usable space in personal vehicles has motivated increased density arrays. Typically, cylindrical cells (having terminals at the ends) are aligned in a number of parallel xe2x80x9csticksxe2x80x9d or end-to-end columns of cells. As well as reducing inactive space, the end-to-end configuration improves spatial density in part by reducing the cell-to-cell electrical interconnection structure. A potential problem with the end-to-end configuration is that significant displacements at the ends of the cell sticks may result from the cumulative thermal expansion in the cells. This thermal displacement, which is the sum of the expansions in the individual cells, must be accommodated by the design of the supporting structure and is a potential source of problems. Even at the highest achievable spatial densities, existing storage devices for automobiles require significant space. Due to their size and power output, such arrays generally require active cooling of some form. One method is to provide a cooling air stream directed through the array and between the sticks of cells.
It is most desirable that a designed cell array be usable in more than one application. Because it is anticipated that personal vehicle designs will continue to show the variability found in the past, cell arrays will be required to fit in various sized and shaped spaces, as well as having various performance requirements. A single fixed cell array configuration is not practical for such a use. An array design that is adaptable to various form factors and performance demands is more desirable. At the same time, cost is always a factor in consumer products, motivating simplicity as well as ease of assembly and repair and replacement.
What is needed is a simplified energy storage device using a cell array design which maximizes cooling efficiency and temperature uniformity to ensure an extended life. At the same time the array design should be easily adaptable to different forms and performance requirements.
The present invention provides an electrochemical cell module including a module housing combinable in various ways and numbers to create different cell arrays. The module housing uses a multi-pass cooling passage that cools cells more efficiently and reduces thermal gradients to increase array life regardless of the number of modules combined in an array.
An object of the present invention is an electrochemical cell module in which cells are retained in a linear side-to-side configuration within a multi-pass cooling passage for effective cooling of the cells and reduced thermal gradients.
A second object of the present invention is an extended life power supply including multiple cell modules each having a reduced temperature gradient.
A further object of the invention is a vehicle powered by an array of electrochemical cells which are cooled in a manner to reduce temperature gradients to provide an extended operational life.
A further object of the invention is a method of ensuring uniform cooling throughout a large array of electrochemical cells by enclosing groups of the cells in side-by-side configurations in individual housings, bundling the housings into a high density configuration and directing a stream of cooling medium through each housing.
A yet further object is an extended life, large, high density array using sub-C configuration cells.
In order to ensure even and uniform cooling of large arrays of primary or secondary cells, the cells are divided into small groups, each group housed and cooled separately. The cells are arranged in side-by-side orientation within module housings. The housings are preferably generally prismatic in shape to ensure compact reassembly into a variety of different configurations. Each housing has a cooling passage extending from a housing inlet to a housing outlet and configured to direct, during use, a cooling medium over the cells. The cooling passage is divided to force the cooling medium in a multi-pass configuration over the cells. The multi-pass passage increases cooling efficiency and reduces thermal gradients throughout the cells. In one preferred configuration, a three-pass passage provides an optimum performance. Because cell operational life is temperature dependent, temperature monitoring devices may be provided on each cell. These can be monitored to detect aberrant conditions which result in elevated temperatures in any one cell.
In a preferred module housing, separated portions of the cooling passage are used to direct cooling medium at the ends of the cells. Conductors are resistance welded to the ends of the cells to provide interconnection. Each housing has a window opening in the top and bottom of the housing to provide access to the cell ends for welding of the conductors. During operation, these conductors act at various times as heaters and heat sinks of the cells. In the preferred embodiment, the conductors reside in the separated portion of the passage to ensure effective cooling. In an alternative embodiment, the passage has a continuously decreasing cross-sectional area resulting in increasing flow velocity along the passage. The increased flow velocity enhances heat transfer to balance the reduced cooling effect resulting from the rise in temperature of the cooling medium.
Large arrays of cells are formed by combining multiple cell modules with their respective inlets and outlets aligned. Various array enclosure plates include raised portions which are sized and arranged to act as inserts fitting sealingly within the window openings of the module housings. The housings are captured between the enclosure plates with the spacing of the inserts fixing the relative position of the housings. The modules are electrically interconnected to provide the output required of the array. Large arrays may consist of one or more vertically stacked decks of modules. In a multi-deck array each deck is joined to the vertically adjacent deck by an enclosure plate having raised inserts on two opposing faces.
The present invention provides novel methods of effectively cooling large arrays of cells to extend their useful operating life. These methods and devices are particularly advantageous to form novel extended-life powered devices such as vehicles. One array, according to the present invention, incorporates sub-C (Cs) configuration cells to form arrays having optimum characteristics for hybrid-electric vehicles, as well as other devices.
Additional advantages of this novel invention as described in the following drawings, detailed description, and claims will be apparent to one skilled in the art.